Kartchner Caverns (the Big Room)

Travel south and east out of Tucson, past adverts on bus-stop benches for the advertising spaces on bus-stop benches, a bikini-topped brunette urging you to EXPOSE YOURSELF DAILY.

Past banners for the upcoming gun fair, now in its fortieth year.

Through the Boneyard and down the Technology Corridor and across the creosote flats.

DRIVE HAMMERED GET NAILED, say the electronic signs spanning the 10.

Then down the 90. Behind you, the Sonoran desert – you’ve not seen a saguaro in miles – and ahead, that stretch of the Chihuahuan Desert that reaches up into Arizona.

On your left, Cochise Stronghold, where the Chiricahua Apache leader defeated and then was defeated by Confederate soldiers, and where, years later, he was buried.

On your right, Cochise Terrace RV Park.

In the distance, over the Huachuca Mountains, hovers a black speck, a remotely-operated dirigible sprouting sensor arrays. The border with Mexico is not far away. (Last time we were out this way, looping back from Tombstone and Bisbee, we got stopped at a Border Patrol roadblock. Fortunately, I am white, speak English and, by chance, had my passport on me.)

Into the misnamed Whetstone Mountains.

The visitor centre is informative, but not dangerously so. There is a short film about the spelunkers discovering the caverns and the fourteen years in which the existence of this network of caves and tunnels and grottoes was kept a secret so as to keep them safe. For some reason, it is scored like a Harry Potter film.

The temperature and humidity hit you the moment you step through the first of six doors and into the first of five airlocks. Outside, it is dry and sunny but chilly in the breeze and cold in the shade; inside, it is warm and wet.

Inside, it is a speleological phantasmagoria.

Stalagmites and stalactites. Soda straws and sodamites [stet]. Vermiform eruptions of gravity-confounding helictites. Like curly fries, or crazy clumpy cartoon wigs caught in a strong wind and instantaneously frozen in their disarray. Dripstone chandeliers. Flowstone like sheets of bacon, like giant Frazzles encased in glossy resin so people will believe my tales of British cuisine in the 1970s. Brushite moonmilk. Totem poles to ancient gods, blurry now. Fried eggs. Popcorn. The colour of strawberries. Anaemic root vegetables. Rust carrots. Ectoplasmic figures as convincing as the luminous muslins extruded by hoodwinking table-rappers must have seemed a century ago. A scarecrow sack-face. Delicate white chocolate confections. Precise one centimetre crenellations. Schools of jellyfish and contorted cephalopods. Rock anemones. A scurrying Nazca spider composed of smudges and folds and cracks. Scallops, their size dependent on the speed and constancy of the flood that swept through and scooped them from the rockface. Candles: some like you find in bistros, the wax overrunning the bottles that hold them; others, stubby and squat, in barely tidy twos and threes as if around an erotic thriller’s bubble bath (you half expect to see a glass of white wine, or some vanilla paraphernalia). A Dimetrodon grandis, turning onto its back as it slowly sink through the rock, breaches this unsuspected world’s stony sky, but all that is left after 270 million years are its spines, fossilised. Around the corner, Dagon towers, distorted, melting, screaming in agony; from its back, a xenomorph’s jaws tear into the air. Near the end, Miss Havisham’s wedding cake opens out like a Rorschach blot, dazzling white, reaching up to the highest point of the Big Room. The female bats give birth there. It is the warmest part of the cave, and it gives newborns their best chance to learn to fly on the one long drop they get. Three hundred thousand years old, they think the caves are, and the bats – no bigger than your thumb – have roosted there every summer for a sixth of that. The oil from their bodies blackens patches of the roof. Their guano, covered in fungus and home to a dozen kinds of troglophile and troglobite, lies in mounds: cavern caviar.

And then it is done. Only three doors and two airlocks, the first of which is musty, like grandma’s fruit cellar, the ranger suggests, but it is more like old books.

It is odd to emerge from darkness into light, to pass from the warm to the cold.